The adjacent_find() function searches between start and end for two consecutive
identical elements. If the binary predicate pr is specified, then it is used to
test whether two elements are the same or not.
The return value is an iterator that points to the first of the two elements
that are found. If no matching elements are found, the returned iterator points
to end.
For example, the following code creates a vector containing the integers
between 0 and 10 with 7 appearing twice in a row. adjacent_find() is then used
to find the location of the pair of 7's: